Project Summary Upper extremity compression neuropathy of the median or ulnar nerve in the veteran population is one of the most frequently encountered disorders seen by hand surgeons. The ability to work and quality of life are often impacted and, in the most severe cases of hand muscle paralysis, adductorplasty and opponensplasty tendon transfer surgeries can be used to restore lateral and opposition pinch function to the thumb. Typically, these surgeries are numerous and developed heuristically with no strong evidence that they are optimally designed to restore large, appropriately-directed thumb-tip force vectors necessary for stable grasps. Surgical outcomes have been mixed. An understanding of the three-dimensional thumb-tip forces normally produced by flexor, abductor and adductor muscles and certain combinations of those muscles could provide a scientific basis for evaluating current surgeries and, if necessary, designing alternative approaches that restore stable lateral and opposition pinch force production. While it is known that muscle thumb-tip force vectors produced by individual muscles vary nonlinearly with muscle/tendon force, the exact nature of the variation is unclear. Moreover, thumb-tip forces produced by groups of muscles and how they change as a function of muscle/tendon forces are unknown. Accordingly, we propose to determine in situ the transformation between muscle/tendon force and thumb-tip force for individual muscles and relevant muscle groups over multiple force levels in lateral and opposition pinch postures. The strength of this proposed pilot work is that it will enable evaluation of current and the design of alternative adductorplasty and opponensplasty surgeries, following peripheral nerve injury, with the intent of improving surgical outcomes in the veteran population.